The field of the invention is fire retardant, fine particulate, expandable styrene polymers for the preparation of molded articles. The present invention is particularly concerned with expandable, particulate molding compositions of styrene polymers containing organic halogen compounds and esters of aliphatic dicarboxylic acids and aliphatic epoxy alchohols.
The state of the art of expandable polystyrene may be ascertained by reference to the Kirk-Othmer, "Encyclopedia of Chemical Technology," 2nd Edition, Vol. 9 (1966) under the section entitled "Foamed Plastics," pages 847-884, particularly pages 852, 853 and 855 where polystyrene is disclosed, and Vol. 19 (1969) under the section entitled "Styrene Plastics," pages 85-134, particularly pages 116-120, where polystyrene foams are disclosed and pages 120, 121 where prior art self-extinguishing polystrene foams are disclosed and U.S. Pat. Nos. 3,755,209; 4,029,614; 4,192,922; 4,228,244 and 4,281,036 the disclosures of which are incorporated herein.
The preparation of the esters of aliphatic dicarboxylic acids and aliphatic epoxy alcohols used in the present invention is disclosed by R. Wegler in METHODEN DER ORGANISCHEN CHEMIE, Vol. XIV, p. 549 as published by Houben Weyl in 1963.
Foamed shaped articles are produced commercially by expanding fine particulate, expandable styrene polymers in molds. In this procedure the fine particulate styrene polymers are first heated with steam or hot gases to temperatures above their softening point, whereby foaming into discrete particles is achieved. This procedure wherein the particles have enough room for free expansion, is termed pre-foaming. The pre-foamed styrene polymers first are stored and then are expanded in a pressure resistant though not gas tight mold by renewed heating with steam, so that the particles fuse together into a shape corresponding to the inside cavity of the mold. This shaping is determined by the spatial bounds of the mold. This second procedure is termed final foaming. The molded object, after final foaming, is cooled inside the mold until the inside temperature drops below the softening point. When the molded object is prematurely removed from the mold, the object deforms. As foam plastics are good insulators, relatively long cooling times are required to cool the mold. The time interval allowing the earliest removal of the molded object without deformation is ordinarily called the "minimum mold dwell time." The drop in the inside pressure to nearly atmospheric can also be used as a criterion for removability from the mold. After being removed from the mold, the molded object typically is stored or aged for some time until cooled entirely and then the shaped articles are cut into foam panels for use as thermal insulation.
Molded products with irregular cell structures are obtained from the final forming of the expandable styrene pre-foamed polymers, especially when a flame retardant halogen compound is added. After having been removed from the mold for a period of time these foamed blocks not only tend toward a substantial collapsing of their sides, called block shrinkage, but also are less firmly fused inside the block. As a result, foam panels cut from such a block are of different qualities. Also, it is necessary to trim the blocks having collapsed sides, and therefore an undesired waste is incurred.
Another problem arises during pre-foaming, because part of the pre-foaming beads shrink, so that low densities desired cannot be controlled. The shrinkage of the prefoaming beads is related to a high loss in expanding agent, whereby the above mentioned uneven fusing and hence the collapse of the block sides are facilitated. Moreover the surface of the finished parts takes on an uneven appearance, which is bothersome especially as regards the manufacture of panels which are visible to the public.
U.S. Pat. No. 3,755,209 discloses that the addition of hydroxylamines to expandable styrene polymers made to be self-extinguishing by organic halogen compounds will improve the above mentioned processing problems. U.S. Pat. No. 4,029,614 describes a similar effect by adding slight amounts of amines free of hydroxyl groups and U.S. Pat. No. 4,192,922 defines amine-substituted triazine derivatives used to remedy these processing problems.
While in many cases all of these additives clearly improve the product properties, they still fail to provide fully satisfactory products. The poor reproducibility of the product properties is most bothersome of all.